Wheelchairs have been designed to provide transportation for the physically impaired, often emphasizing user comfort, portability and flexibility. Because of the individual needs and requirements of the wheelchair occupants, a variety of styles and shapes have been developed which cater to their specific needs. Thus, both manual and power driven wheelchair apparatus have become significantly more sophisticated during the past decade.
One advantageous feature common to both manual and power driven wheelchair apparatus is to provide a backrest portion of the seat support which moves pivotally between a generally vertical position and a generally horizontal position (forwardly or rearwardly) parallel to the seat. This arrangement provides either user comfort or provides for partial collapsibility of the wheelchair frame to reduce the overall dimensions for storage or transportation. Often, the backrest portion may be reclined substantially rearwardly while the occupant remains in the wheelchair for therapy or increased accessibility during examinations.
One problem associated with these pivotal backrests during reclination, however, is that body shear forces are produced on the back and the buttocks of the wheelchair occupant. Typically, a backrest pad is fixedly mounted to a backrest frame which is pivotally mounted to the wheelchair main frame about a generally horizontal axis. When the backrest frame is reclined with the wheelchair occupant still seated in the wheelchair, the backrest pad slides along the occupant's back in a direction toward their head causing body shear.
Attempts to overcome this problem usually include cooperating mechanisms formed to coordinate travel of the back assembly with the forward travel of the seat assembly. Generally, a wheelchair is provided including a pivotal backrest assembly having a backrest portion formed for sliding movement longitudinally along the backrest frame in coordination with the reclining of the backrest frame. Typical of these patented sliding backrest devices are the assemblies found in U.S. Pat. Nos. 5,320,412; 5,044,647; and 4,655,471.
While these assemblies have been adequate to reduce the shear forces between the occupant's back and the backrest assembly, they generally include a complicated assembly of interengaging linkages which are difficult to assemble and costly to manufacture. Moreover, a portion of these linkages often must be positioned beneath the seat assembly which causes the overall height of the wheelchair to be increased.